Petr N., a Czech guy who also sent me the first e-mails about the 9/11 attacks 30 minutes before my PhD defense in New Jersey began at 9:30 a.m. in 2001, informed me about a wonderful new story in numerous media, a story about the Chinese quantum radar.
For example, some journalists in New Zealand boldly claim:
hit an overpriced F-35 project with a thermonuclear tweet. Before he tweets again and demands the Lockheed-Martin bosses to commit harakiri because of the amazing achievement by the Chinese, I urge him to think twice and read this blog post.
Czech L-159's – used by the Iraqi Air Force along with some F-16's – are almost an order of magnitude cheaper than F-35's but they're still credible aggressor fighters. Too bad that the Donald can't import things from his first wife's homeland.
There could be better radars that could be called "quantum radars" for one reason or another but the claims about the "quantum radar" turn out to be based on a paper written by authors who completely misunderstand quantum mechanics, e.g. crackpots. Because the authors are Chinese, they must be classified as Chinese crackpots.
The reason why these people are crackpots has been discussed many times on this blog, e.g. here. As you may learn in this article and others, the key paragraph in the paper underlying the Chinese "quantum radar" is the following:
“Quantum physics says that if you create a pair of entangled photons by splitting the original photon with a crystal, a change to one entangled photon will immediately affect its twin, regardless of the distance between them,” the paper states.Too bad, quantum mechanics says nothing of the sort.
These Chinese comrades basically believe – much like Mark Alford, Jean Bricmont, and other crackpots in the present and the past, basically including John Bell – that the quantum entanglement implies that there exists a remote and instantaneous transfer of the information from one member of an entangled pair to the other member. When something happens to the remote twin brother (including events that were decided by a human), something analogous immediately happens to the twin brother inside the radar as well and it may be seen.
But that's not what can happen in Nature. If some collision of the remote photon with an F-35 influenced how our local photon responds in the radar (e.g. if it got reflected from the air/vacuum in the "analogous direction" and painting an image of the airplane in a detector), it would be a non-local interaction – an action at a distance. But since 1905, we've known that non-local itneractions and the action at a distance is strictly prohibited in Nature. Any transmission of the information that would be faster than the speed of light could be interpreted as the transmission of information from the present to the past – it's enough to Lorentz-boost the situation and describe it from a different reference frame where the chronology of the spacelike-separated events is reversed. But the influence on the past would be a clear logical contradiction because you could castrate your grandfather before he met your grandmother, and your existence would therefore be an oxymoron (that's why all closed time-like curves are prohibited in Nature).
Consequently, in every consistent theory invariant under the Lorentz transformations (including the boosts), influences may propagate at most by the speed of light.
Quantum entanglement doesn't change an epsilon about these principles of relativity. When we create an entangled pair, the properties of both members of the pair are correlated. But they are not correlated because of some non-local, superluminal, or instantaneous interaction at the moment when these two members of the pair are far from one another. They are correlated simply because they were correlated from the beginning – they were created at one place and one moment in a way that guarantees this correlation (e.g. because of the angular momentum conservation law, the spins of the twins are perfectly anticorrelated).
When the polarization of one of the photons is measured by Alice, she becomes aware of the polarization of her photon but – because of the perfect anticorrelation of the spins – also about the polarization of the other photon that will be measured by Bob. So Alice knows what Bob will measure (or has measured). But she cannot influence what Bob will measure because one bit (e.g. Alice's photon's spin) that comes from the measurements is truly and perfectly random. The random generator is run by Mother Nature and no human can influence the outcomes by his or her will. The "relative spin" (whether the spins are equal) is determined from the beginning but one of the spins (e.g. Alice's photon's spin) is not – it is truly random. There is no contradiction in these statements.
It's easy to prove within a realistic, local quantum mechanical theory – e.g. within any quantum field theory – that all the measurements of the photon inside the "radar" done by Alice – a U.S. liberal and Hillary voter who emigrated into China, works with Chinese radars, and is trying to damage the U.S. Air Force – will be unaffected by any interaction that the other photon underwent, and any choice of the "kind of measurement" that Bob – who is a pilot of an F-35 – could have picked by his will. There will always be correlations between the outcomes of "analogous" measurements performed on both photons but these outcomes cannot be determined by any person's will or by any collision with an aircraft, among other events. The two photons' spin are anticorrelated but otherwise perfectly random.
So if Alice measures e.g. the circular polarizations of the members of many entangled pairs that flew into or stayed in the Chinese radar, she can be sure that 50% of these photons will be left-handed and 50% of these photons will be right-handed (if the number of photons is statistically large). And it will be random which of them are left-handed and which of them are right-handed – there will be no patterns. This fact can in no way be affected by any collision of the remote member of the entangled pair with a U.S. aircraft or by any decision of a U.S. pilot.
The basic principle of the "quantum radar" is nothing else than the standard crackpottery parroted by all the people who totally misunderstand what the quantum entanglement actually is and how it works. Also, almost all the criticisms in the media are rubbish. Some people say that the "quantum radar won't work because of decoherence". Decoherence is a fashionable buzzword among people who want to sound smarter than they are but they're also stupid because the reason why the quantum radar cannot work has nothing whatever to do with "decoherence".
It's obviously possible that the paragraph about the action at a distance is just a mist written in order to make the Chinese quantum radar stealth and the actual way how the radar is working is completely different. If that's so, the Chinese quantum radar may be a real thing, of course. But if the quantum radar is really supposed to work on the interpretation of the quantum entanglement that I quoted, I may assure you that the people who "work" on this idea are crackpots and imbeciles of the very same kind as Jean Bricmont, Mark Alford, and many others – including many authors of anti-quantum "popular" books (especially all the "Bohmists").
There is no action at a distance in Nature. Einstein chose the term "spooky action at a distance" for the quantum entanglement but that was because he didn't understand quantum mechanics himself. The quantum entanglement, while a totally omnipresent feature of Nature as predicted by quantum mechanics, is neither spooky nor an action at a distance. We sometimes use the phrase "spooky action at a distance" as a synonym of the quantum entanglement but when we do so, we just shouldn't forget that this phrase is absolutely misleading as every level, just like if you used the term "a white sheep" for a black hole. You could adopt this terminology for fun – or to celebrate a physicist – but you should never forget that a black hole is neither white nor a sheep. It's a trap! (And the previous sentence is true in both interpretations of "it"! "It" may be the black hole or the sentence.) Consequently, quantum mechanics doesn't allow any "quantum radars" that would require a genuine action at a distance.
There may be lots of reasons why the U.S. military should leave F-35 but the Chinese quantum radar based on the non-local interpretation of the quantum entanglement isn't one of them.